Blender Camera Tracking ground plane - unable to adjust shadow darkness
Just to cover the basics first, check the ground plane material; the material settings will affect the shadow's visibility.  Also, if the world is emitting light, that can dim the shadow. Color space can affect it; sometimes filmic will dim it even further in the compositor.

If not, the issue is likely from the render layer's alpha channel.  You can change the alpha's strength with a color ramp or multiply node.  Feed that ramp with the render into a 'Set Alpha' node, which you can then feed into the alpha over node.